All the discussion on AGC got me interested in finding out why the IC-781
has the most transparent AGC action I've found among all the radios I've
used. By transparent, I mean it doesn't get in the way of copying signals
under a wide range of conditions. With other radios, I'm always aware that
there is an AGC at work and sometimes it actually impairs copy.
Furthermore, in the 781 there seems to be what various people describe
qualitatively as depth or resolving power. You can listen into a pileup of
multiple signals and pull out individual signals more clearly than in other
radios, or similarly pull weak signals out of a high background noise,
whether it's static crashes or smooth white noise.
What I found through measurements is that the AGC threshold in the IC-781
is about 4 uV (measured on 160m) with the RF gain at maximum. Below the
threshold there is a 1:1 change in audio output with signal input level.
Above threshold, the AGC clamps the audio output at a constant level with no
measureable change in audio output for increasing signal input. There is
definitely no gain "slope" and no other obvious magic.
What I'm led to conclude is that it's the AGC dynamics (i.e. attack and
decay behavior, hang time, etc.) that must account for at least part of what
I'm hearing. One feature in the 781 which doesn't exist in any other radio
that I'm aware of is a continuously variable AGC time constant. I find this
feature alone to be very effective in tailoring AGC action precisely to the
band conditions. For weak signals in static crashes, I run the shortest
time constant so the radio recovers quickly between crashes, which it does
without pumping. For pileups, it helps to increase the time constant to
reduce gain modulation, which occurs when a strong signal "captures" the AGC
and makes it more difficult to copy other signals as the gain is being
modulated by the strong signal.
I'm not saying that the AGC can't be improved, perhaps by introducing gain
slope, but I am pretty satisfied already with the way it is now. By the
way, these observations are based on years of specializing in lowband
contesting in the major DX contests, which produce some of the most
stressful conditions for evaluating radios.
Once my new Orion arrives (it's just been shipped and I'm waiting for the
UPS delivery), I intend to play some more with its programmable AGC
parameters to determine if I can improve upon what I observed (and reported
here) in the CQ160 contest. What I do know about the Orion is that its
default AGC settings are not optimal for 160m.
73, John W1FV
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